Quasi-epitaxial Metal-Halide Perovskite Ligand Shells on PbS Nanocrystals

Epitaxial growth techniques enable nearly defect free heterostructures with coherent interfaces, which are of utmost importance for high performance electronic devices. While high-vacuum technology-based growth techniques are state-of-the art, here we pursue a purely solution processed approach to obtain nanocrystals with eptaxially coherent and quasi-lattice matched inorganic ligand shells. Octahedral metal-halide clusters, respectively 0-dimensional perovskites, were employed as ligands to match the coordination geometry of the PbS cubic rock-salt lattice. Different clusters (CH₃NH₃ ⁺)_(6-x)[M^(x+)Hal₆]^(6-x)- (M^x+ = Pb(II), Bi(III), Mn(II), In(III), Hal = Cl, I) were attached to the nanocrystal surfaces via a scalable phase transfer procedure. The ligand attachment and coherence of the formed PbS/ligand core/shell interface was confirmed by combining the results from transmission electron microscopy, small-angle X-ray scattering, nuclear magnetic resonance spectroscopy and powder X-ray diffraction. The lattice mismatch between ligand shell and nanocrystal core plays a key role in performance. In photoconducting devices the best performance (detectivity of 2 × 10¹¹ cm Hz ^1/2/W with > 110 kHz bandwidth) was obtained with (CH₃NH₃)₃BiI₆ ligands, providing the smallest relative lattice mismatch of ca. ∼1%. PbS nanocrystals with such ligands exhibited in millimeter sized bulk samples in the form of pressed pellets a relatively high carrier mobility for nanocrystal solids of ∼1.3 cm²/(V s), a carrier lifetime of ∼70 μs, and a low residual carrier concentration of 2.6 × 10¹³ cm^-3. Thus, by selection of ligands with appropriate geometry and bond lengths optimized quasi-epitaxial ligand shells were formed on nanocrystals, which are beneficial for applications in optoelectronics.